1. Field of the Invention
The present invention relates to an improvement of a fixing roll for thermally fixing an unfixed picture image or the like in an electrophotographic reproducing machine or an electronic printer to a recording material such as a paper sheet.
2. Description of the Related Art
FIG. 2 shows a fixing roll of a high durability, which is widely used in recent years in an electrophotographic reproducing machine. As shown in the drawing, the conventional fixing roll comprises a core metal 1, a heat resistant elastic layer 2 formed to cover the circumferential surface of the core metal 1, and a surface layer 3 consisting of PFA resin (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer). The surface layer 3 is formed in a thickness of tens of to hundreds of .mu.m.
The PFA resin forming the surface layer 3 exhibits an excellent release property and, thus, permits effectively suppressing the tonor off-set problem, i.e., the problem that the tonor is attached to the surface of the fixing roll in the thermal fixing step. Also, the PFA resin surface layer is thin, i.e., not thicker than hundreds of .mu.m. In addition, a heat resistant rubber or a heat resistant sponge used for forming the heat resistant elastic layer 2 is soft. The small thickness of the PFA resin surface layer and the softness of the heat resistant elastic layer 2 are combined to permit a sufficient nipping width required for the thermal fixing, with the result that the fixing roll can be used satisfactorily over a long period of time.
However, the PFA resin is a high insulator having a volume resistivity of at least 10.sup.16 .OMEGA..cm and, thus, tends to be charged easily. When used for forming the surface layer of a fixing roll, the PFA resin is frictionally charged up to scores of volts to scores of thousands of volts by the contact with a copying paper in the thermal fixing step or with another roll or belt. Because of the charging to a high voltage, tonor or paper dust is electrostatically attached to the fixing roll surface, with the result that the high release property of the PFA resin is impaired so as to bring about the tonor off-set problem noted above.
For solving the above-noted problem brought about by the electrostatic charging, it is proposed to lower the volume resistivity of the heat resistant elastic layer 2 so as to suppress the electrostatic charging on the surface of the fixing roll. However, it is impossible to suppress the charging sufficiently by lowering the volume resistivity in the case where the thickness of the surface layer 3 formed of the PFA resin is increased. It is also proposed to suppress the electrostatic charging by lowering the volume resistivity of the PFA resin layer 3. It may be reasonable to add carbon black or metal powder to the PFA resin in order to lower the volume resistivity of the PFA resin layer 3. In this case, however, the high release property of the PFA resin is impaired, leading to a tonor off-set problem in a short time.